I disagree, I believe all of the roles can both carry and cost you. Support is fun in that unlike other lanes, you don't have to focus on farming much early and are instead focusing on engages with the enemy players immediately with a sub-focus on map awareness. You have a mixed lane/teamfight element immediately and are generally the aggressor/initiate of your pair.
OP is onto something with his thinking. Basically, ADCs are very damaging but easy to kill. While positioning is important for every role in League, there is no other role that depends on it so absolutely critically as ADC. It's not just about kiting and peel (though you need those too), but also simply where you are at all times. This is also part of the reason champs like Sivir (my personal favorite) or Ashe are a bit harder to use than Caitlin or even Tristana. Sivir has short range and Ashe has no escape, so champs like that live (and often die) by their positioning, but you also need to position to be able to deal damage. Other champs need better timing, reaction to enemy skills, or careful prioritization of targets. ADC needs razor-thin margin of positioning.
ADCs, with a few exceptions, are most certainly NOT duelists. Any similarly farmed assassin or bruiser can 1v1 you easily, but if they're cc'd for even a moment if you're farmed like you should be your own damage can blow them (and their team) up instead.
